The P/E Shortcut for Accretion/Dilution: Answer in 10 Seconds

You're in a superday and the interviewer leans back: "Company A trades at 20x earnings and buys Company B for 16x earnings in an all-stock deal. Accretive or dilutive?"

You don't need a merger model. You don't even need a calculator. If the buyer's P/E is higher than the P/E it pays for the target, an all-stock deal is accretive. That's the P/E shortcut, and this post shows you why it works, how to extend it to cash and debt deals, and where it breaks down.

Quick Refresher: What Accretion/Dilution Means

A deal is accretive if the buyer's earnings per share (EPS) go up after the acquisition. It's dilutive if EPS go down.

That's it. We're comparing the buyer's standalone EPS to its pro forma (combined) EPS. The shortcut is just a fast way to predict which direction that number moves.

The Core Idea: Earnings Yield vs. Cost of Financing

Flip P/E upside down and you get earnings yield (E/P). A company at 20x P/E has an earnings yield of 1/20 = 5%.

Now think about what happens in an acquisition:

• What you get: the target's earnings. Divide them by the price you pay and you get the target's earnings yield.
• What you give up: whatever it costs to fund the purchase.

If the earnings you buy are bigger than the cost of the money you use, the deal is accretive. Everything else is just figuring out the "cost" of each funding source.

The cost of each funding source

Why is stock's cost the buyer's earnings yield? Because new shares give away a slice of every future dollar of earnings. A buyer at 20x is effectively "paying" 5 cents of earnings for every dollar of stock it issues.

Worked Example: The All-Stock Deal

Let's set up two companies.

Acquirer

• Share price: $50
• EPS: $2.50
• P/E: 20x
• Shares outstanding: 100 million
• Net income: $250 million

Target

• Net income: $50 million
• Purchase price (equity value, including premium): $800 million
• Purchase P/E: $800m / $50m = 16x

Run the shortcut

Acquirer P/E (20x) > purchase P/E (16x). Accretive.

Prove it with the math

• New shares issued: $800m / $50 = 16 million
• Pro forma net income: $250m + $50m = $300m
• Pro forma shares: 100m + 16m = 116m
• Pro forma EPS: $300m / 116m = $2.586

That's up from $2.50, about 3.4% accretive. The shortcut called it.

What if the buyer overpays?

Raise the purchase price to $1,200 million (24x the target's earnings):

• New shares: $1,200m / $50 = 24 million
• Pro forma EPS: $300m / 124m = $2.419

Down about 3.2%. Dilutive, because 20x < 24x.

And at exactly $1,000 million (20x)? You issue 20 million shares, and $300m / 120m = $2.50. Breakeven. When the two P/Es match, EPS doesn't move.

Extending the Shortcut to Debt and Cash

Interviewers love the follow-up: "What if they use debt instead?" Same logic, different cost.

All-debt example

Assume the acquirer borrows at 6% with a 25% tax rate.

• After-tax cost of debt: 6% × (1 − 25%) = 4.5%

Compare that to the target's earnings yield at each price:

Let's check the $1,200m case:

• Interest: $1,200m × 6% = $72m
• After-tax interest: $72m × 75% = $54m
• Pro forma net income: $250m + $50m − $54m = $246m
• Shares: still 100m (no new stock)
• Pro forma EPS: $2.46, dilutive. ✅

Notice something interesting: at $1,000m, the all-stock deal was breakeven but the all-debt deal is accretive ($255m / 100m = $2.55). That's because debt (4.5%) is "cheaper" than this buyer's stock (5%). This is a classic interview insight.

Mixed financing

For a combination, just take the weighted average cost.

Say the $1,200m deal is 50% stock, 50% debt:

• Weighted cost: (50% × 5.0%) + (50% × 4.5%) = 4.75%
• Target yield: 4.17%
• 4.17% < 4.75%, so dilutive

Quick check: $600m of debt costs $27m after tax, $600m of stock means 12m new shares. Pro forma EPS = ($300m − $27m) / 112m = $2.44. Dilutive. ✅

How Much Synergy Fixes a Dilutive Deal?

This is a great way to stand out. Take the dilutive all-stock deal at $1,200m:

• To hit breakeven EPS of $2.50 with 124m shares, you need $2.50 × 124m = $310m of net income.
• You have $300m, so you need $10m of after-tax synergies.
• Pre-tax, that's $10m / 75% = about $13.3m.

Saying "this deal needs roughly $13 million in pre-tax synergies to break even" sounds a lot more like a banker than just "it's dilutive."

Where the Shortcut Breaks Down

The P/E shortcut is a first pass, not a full merger model. It ignores:

• Synergies: cost savings or revenue gains push deals toward accretion.
• New D&A from purchase accounting: writing up the target's assets and creating amortizable intangibles adds expenses that reduce pro forma earnings.
• Transaction and financing fees: these hit the numbers too.
• Mismatched time periods: compare forward P/E to forward P/E, or trailing to trailing. Mixing them gives garbage.
• Using the wrong target price: the purchase P/E must use the offer price (including premium), not the target's current trading price.

If an interviewer asks a straightforward question, give the shortcut answer. Then mention one or two of these caveats to show you know its limits.

One Last Thing: Accretive Doesn't Mean Good

A high-P/E company can make almost any all-stock deal accretive, including terrible ones. EPS accretion says nothing about whether the buyer paid a fair price or whether the deal creates real value.

If you can add that nuance in an interview, along the lines of "it's accretive, but accretion alone doesn't tell us if it's a smart deal," you'll come across as someone who understands why the math matters, not just how to do it.

Your Cheat Sheet

• All stock: buyer P/E > purchase P/E means accretive.
• General rule: target earnings yield > weighted after-tax cost of financing means accretive.
• Cost of stock = 1 / buyer P/E. Cost of debt = rate × (1 − tax). Cost of cash = foregone interest × (1 − tax).
• Equal yields mean breakeven.
• Always caveat: synergies, new D&A, fees, and matching time periods.

Practice this with a few made-up numbers until you can do it out loud in under 30 seconds. It's one of the most reliable technical questions you'll face, and now you've got the shortcut down cold.